Arc discharge apparatus



Dec. 18, 1934. A. GLASER ET AL 1,985,087

I ARC DISCHARGE APPARATUS Filed Nov. 4, 1952 GRAPHITE OXIDE- COATEDCATHODE 1 I ll L 7 I 5 E: a

Ihvehtofs August Glaser; Welrheir- Koch 9 Their Attohhez Patented Dec.18, 1934 UNITED STATES PATENT OF'FIICE ARC DISCHARGE APPARATUS AugustGlaser, Berlin-Frohnau, and Werner Koch, Berlin-Hermsdori', Germany,minors to General Electric Company, a corporation of New YorkApplication November 4, 1932, Serial No. 641,264 In Germany November 9,1931 2 Claims.

In arc discharge devices which employ grids,

there is a normal tendency for the control factor to vary. Thesevariations have been found to reside for the most part in the griditself and appear to have their inception at the surface of the controlmember. In a device containing gas or vapor, and an oxide-coatedcathode, directly or indirectly heated, the oxide tends to sputter orevaporate from the cathode. If the grid is made of a metal, such asnickel, which readily combines with the oxide, the active material fromthe cathode is deposited on the grid and emits electrons.

An object of our invention is to improve the operating characteristicsof grid controlled arc discharge devices and in particular, to improvethe control member of these devices which contain oxide-coated cathodes.Briefly stated, and in accordance with our invention, we propose tosubstitute solid graphite for the ordinary nickel grids. The inventionwill be better understood when reference is made to the drawing whichshows an elevational view, in perspective and partly broken away, of atube embodying our invention. The figure shows a conventional form oftube, apart from the improved grid, which is described by A. W. Hull inarticles entitled Hot Cathode Thyratrons" appearing in the GeneralElectric Review, Vol. 32, 1929, at pages 213 to 223 inclusive, and 390to 399 inclusive. In the drawing, numeral 1 designates an envelopecontaining a cathode 2 of the well-known indirectly heated type,surrounded by a perforate grid 3. The grid is open at the bottom andclosed at the top except for the discharge openings. The cathode issupported by leading-in conductors 4 sealed in the lower stem 5 andextended through the envelope to the terminals 6. A pair of theseconductors serves to carry the load current and the other conductor isconnected to the heater within the cathode. The cathode consists of ametal cylinder, preferably nickel, with vanes extending radiallyinwardly to support the cylinder from the inner core which contains theheater. The exterior of the cylinder, and the spaces between the vaneson the interior of the cylinder are coated in any suitable and wellknownmanner with electron-emitting material, preferably barium and its oxide.The grid 3 derives its support from a metal clamp 7 which embraces thestem 5. A conductor 8 is connected between the clamp and the middleterminal member 9. The anode 10, which may be fabricated of carbonizedmetal or solid graphite, takes the form of a circular plate presentedfiatwise to the grid and is supported from the other end of theenvelope. The envelope contains an inert gas, such as helium or argon ata pressure, for

example, 500 microns, sufficient to support an arclike discharge at theoperating voltages.

As stated in the articles referred to, devices of this sort arepreferably energized by alternating current. The starting of the arc maybe controlled by alternating or direct current voltages applied to thegrid and since the arc is stopped by the periodic removal of the anodevoltage, 1. e. at the end of each positive half-cycle, the griddetermines the average value of the rectifier current output. Inpractice, it is desirable that the control exercised by the grid, i. e.the determination in each half-cycle of the anode voltage at which thearc shall start, be uniform throughout the recurring cycles. As statedhereinbefore, considerable variation in this control has beenexperienced, particularly in those tubes which employ oxide-coatedcathodes. The tendency of the control factor to vary under theseconditions has been traced to the grid, and it is found that the gridemitted electrons and was taking over temporarily, the function of acathode. Generally speaking, the higher the operating temperature of thetube and grid, the greater is this emission effect. When the grid memberis made of nickel, and an alkaline earth oxide such as barium oxide, isemployed as the active coating on the cathode, such oxide particles asleave the cathode become deposited on the grid due to the large aflinitythat nickel has for the oxide. Other grid metals having less affinityfor the oxide material than nickel also form an oxide deposit to asmaller degree. This deposit or layer is most pronounced at the portionof the grid which lies directly in the electron path.

In order to eliminate these variations of control, we propose to usegraphite for the control member, the graphite being in the solid form asdistinguished from simply a carbon coat or paint. A grid made of thismaterial may be fabricated in any well-known and suitable manner andprovided with apertures for the discharge current, similar to theordinary metal grids. Due to the fact that graphite has little or noaffinity for barium oxide, the charged oxide particles do not lodge toany great extent thereon and such few particles as do stick, offer noharmful efiects from the electron emission standpoint. The use ofgraphite for this purpose presents an'advantage over the ordinary metalgrids in that it radiates heat to a greater extent on account of theincreased black body effect and hence, operates at a lower temperaturefor a given bulb temperature. By lowering the temperature 01' the grid,the tendency to emit electrons is still further decreased. Graphite insolid form is particularly advantageous over carbon-coated grids in thatit not only offers greater black body ell'ect as a whole, but also doesnot scale or peel as would be the case simply of a coating.

Inasmuch as the collection of the oxide particles takes place to agreater extent at that portion of the grid which is exposed directly tothe electron stream, i. e. the portion interposed immediately betweenthe cathode and anode, it may be desirable to have only this portionmade of graphite and the remaining portion, 1. e. the part whichsurrounds the cathode, constituted of metal in the usual manner. In thiscase, the advantages of graphite insofar as ofiering freedom fromcontrol disturbances is concerned, are not obtained in the fullestdegree but from a practical standpoint, the compromise structure may beall that is necessary for a given tube performance.

While we have described our invention with particular reference to anindirectly heated cathode, it is apparent that the invention is notlimited thereto but contemplates the use of all types of oxide-coatedmembers, filaments, etc., indeed, to any form of cathode from whichelectronically abtive material is evaporated during operation.

What we claim as new and desire to secure by Letters Patent 01. theUnited States, is:

1. An electron discharge device comprising an envelope containing anoxide-coated cathode and an anode in opposed relation, a gas in saidenvelope at a pressure sufllcient to support an arc-like discharge atthe operating voltages, a metal grid surrounding said cathode and havinga portion disposed between the cathode and anode, said grid portionbeing constituted of graphite.

2. An electron discharge device comprising an envelope containing acathode and an anode, a gas in said envelope at a pressure sufiicient tosupport an arc-like discharge at the operating voltages, a cylindricalgrid of metal surrounding the cathode and having a transverse portionconstituted of graphite.

, AUGUST GLASER.

WERNER KOCH.

